Fix the support for trusted DHCP server in the ebtables code's
hard-coded function applying DHCP only filtering rules:
Rather than using a char * use the more flexible
virNWFilterVarValuePtr that contains the trusted DHCP server(s)
IP address. Process all entries.
Since all callers so far provided NULL as parameter, no changes
are necessary in any other code.

There is something I don't understand in that loop, you repetedly
write to buf, but you don't seems to use buf in the loop. This looks
fishy to me, or are you using side effect execution in the Asprintf
argument evaluation. Too cryptic to my taste, I'm lost !

I am accumulating (shell) commands in the buffer and then test the
buffer for error before finally executing the commands.